The present invention relates to electric water heaters and in particular to detecting the presence of entrapped air which may cause a dry fire.
Portable spas often use electric spa water heaters. Such heaters include a heater element(s) intended to be completely immersed in water while the heater is operating. Unfortunately, in some instances air may become entrapped in the heater housing, and energizing the heater while air is entrapped in the heater housing results in overheating and possibly destroying the heater element. Because air has a much lower density then water, when the heater is energized while a portion of the heater element is exposed to air, air temperature will be increased rapidly in the heater housing at a rate of 1000 degrees per seconds. Such overheating is the most frequent cause of heater failure in hot tub applications.
The heater element comprises an inner wire separated from an outer tubular wall by insulating material, for example, Magnesium Oxide (MgO) insulating material. The outer tubular wall is generally connected to ground. A rapid increase of temperature of even a portion of the heater element destroys or damages the insulating material causing current leakage from the inner wire to the outer wall of the heater element and shorting to ground. The resulting failure is referred to as heater dry fire.
Many devices have been made trying to prevent heater dry fire by detecting to the present of fluid inside the heater housing. Examples of such devices include vacuum switch, pressure switch, flow switch, and temperature sensors which monitor the rapid increase of temperature in the heater housing. None of these devises have provided adequate protection to the heater element from dry fire.
Unfortunately, operation of a low speed pump may not provide sufficient water flow rate to push an air pocket through the heater housing and may only provide a flow below the air pocket. As a result, the air pocket may be entrapped against the highest interior surface in a heater housing. Vacuum and pressure sensors may fail to detect the air pocket because the fluid (a combination of water and air) inside the heater housing creates enough pressure or vacuum to close the pressure or vacuum sensors circuitry.
For example, as the low speed pump pumps water through the heater housing, the pump would create pressure inside the heater housing, even when air is entrapped inside the housing, where a pressure switch is mounted. Known pressure switches, at as low as 1.5 PSI, send a signal to a heater control circuit to activate the heater. The heater control circuit turns the heater “on”, heating the water circulating through the heater. Entrapped air inside the heater housing is compressed resulting in uniform pressure in both the water and entrapped air. If the heater comes “on” when air is present inside the heater housing, the heater (i.e., the pressure switch) cannot distinguish between water pressure and air pressure, resulting in heater damage almost immediately, for example in less than 10 seconds, depending on heater wattage.
U.S. Pat. No. 8,406,932 to Hollaway discloses a heater including a sensor which monitors an abnormal rise in temperature in the heater housing. Unfortunately, this method is not be effective at all for many reasons:
1—The sensor(s) can only detect the rise in temperature after the heater is turned on. When the heater comes on with air trapped in the heater housing, the heater internal insulation is destroyed or damaged by the time the sensors detect the rapid rise in temperature and turn off the heater.2.—The temperature sensors cannot distinguish between air and water temperature in the heater housing. They can only turn the heater “On” or “OFF”. The electric heater can be heating air, water or a combination of air and water
U.S. Pat. No. 7,791,004 to Reusche discloses temperature sensors and a fluid sensor attached to a water heating element insertable into a bucket of water. Unfortunately, the fluid sensor only detects the absence of water in a particular location and would fail to detect entrapped air residing against the ceiling of a water heater housing.